This invention relates to a broadcast data transmission system, and more particularly to a broadcast data transmission system providing for data flow control.
Various modes of data transmission systems have been employed for local area networks (LANs). One of these is the broadcast data transmission system. As shown in FIG. 1, a LAN has a number of stations 11 that are connected by a transmission line 12. Data to be transmitted from an internal buffer 13 of one station 11 is sent on transmission line 12 as broadcast data. Each station 11 in the LAN then receives the broadcast data and processes it, storing the boardcast data in its own internal buffer 13.
Items of broadcast data are transmitted and received by stations 11 in the format of transmission frames. In broadcast data transmission systems, transmission frames do not include destination address information for the broadcast data. Therefore, the station that transmitted a broadcast data item also receives the transmission frame containing that item of boardcast data.
In this way, broadcast data can be transmitted to all stations 11 in the LAN at once in a broadcast data transmission system. Compared with systems in which data must be transmitted separately to each station, data transmission efficiency is greatly increased in the broadcast data transmission system.
Unfortunately, there are problems associated with the use of broadcast data transmission systems. FIG. 2 shows the amount of traffic on transmission line 12 at certain times t. The traffic amount number Nt corresponds to the number of items of broadcast data present on the transmission line during a unit time period (having a duration of 1.0 second in the example shown in FIG. 2) at a particular time. As shown in FIG. 2, at certain times t the traffic amount number Nt associated with that time t is not less than a traffic limit number, designated as Nm. During periods of time when the amount of traffic reaches the traffic limit, as shown by the shaded region in FIG. 2, the reliability of a conventional broadcast data transmission system may be degraded.
Each station 11 in the LAN receives and stores broadcast data in its internal buffer 13 and needs time to process the stored broadcast data. As the amount of broadcast data sent on transmission line 12 during a time period increases, it becomes more difficult for the stations to process the received broadcast data. If broadcast data is received without any flow control on the traffic amount, at some point the traffic amount will be so heavy that the maximum amount of data that can be processed in the internal buffers of each station 11 will be exceeded.
When processing by a station cannot keep up with rate at which new broadcast data is being received and stored, the internal buffer for that station will remain in a busy state. If its internal uffer 13 remains busy, station 11 will not be able to receive additional broadcast data on the transmission line. Therefore, some flow control on the traffic is needed. However, in a broadcast data transmission system, the sending station essentially requires no answer or acknowledgement from the receiving stations, thereby preventing the sending station from determining whether the internal buffers in the receiving stations are busy. Consequently, it is extremely difficult to provide a broadcast data transmission system in which sending stations can exercise data flow control over the sending of broadcast data on the transmission line.
Therefore, in order to improve the reliability and efficiency of data transmission in broadcast data transmission systems, there is a need for data flow control by the sending stations in such systems.